Intensity modulated direct detection (IMDD) with 4 level pulse amplitude modulation (PAM-4) has been adopted as a standard for high speed short reach 400 Gbps Ethernet applications. IMDD PAM-4 can be used over a single mode fiber at 56 Gbps for reaches from 2 km to 10 km. However, if the required transmission length is extended beyond 10 km, chromatic dispersion becomes a limiting factor.
It is desirable to extend the transmission distance of IMDD PAM signals.